1. Field of the Invention
The present invention relates to a method and apparatus for detecting and/or imaging an object.
2. Related Art
Image through turbid media is often carried out using Time of Flight measurements. Various methods of time of flight measurement are known. The majority of these methods measure the time taken by pulses of light to travel a return path. Since the velocity of light is both constant and known for most materials, measurements of time of flight can be readily converted into images of the medium through which the light has passed.
Time of Flight imaging is particularly useful when imaging over large distances or in semi-turbid media. However, when dealing with short distances image resolution decreases rapidly, since the speed of light is so great that, for example, a spatial resolution of 1 m will require a detector with a temporal resolution of 5 ns. The shortest measurable distance and the image resolution obtainable using Time of Flight imaging is thus limited by the response time of the detectors used. A further disadvantage or Time of Flight imaging is that the reset time of the detectors used is considerably longer than the jitter time, so that the detectors are only capable of detection for a very short period of their total operating time. An alternative method of imagine through turbid media is it use acoustic waves (eg. ultrasound imaging). However, acoustic imagine suffers from lack of spatial resolution due to the large divergence of acoustic waves.
A further known method of imaging comprises forming an image of an object from interference of two beams of coherent light, one of which has been scattered from a target (ie. holographic imaging). Holograms have been used to analyse non-visible parameters of a target, for example, vibration of an engine block. Holographic imaging suffers from several disadvantages Firstly, holograms require two investigative beams that must interfere and be coherent over the distance to the target. Secondly, holograms are not suited to imaging through opaque media. An image cannot therefore be produced at a distance greater than a single photon transport path of the light used to obtain the hologram. Thirdly, holograms arc unsuited for measurement through media that exhibit dynamic scattering, and the scattering will reduce the quality of images obtained.
Several known imaging techniques exist where an investigative wave is perturbed as it passes, scatters, reflects or is absorbed by a target. These techniques require that the form of energy used for the investigative wave, and its frequency, must be chosen to interact with the target and cannot thus be fully optimised for detection (i.e. low absorption an/or high spatial resolution and/or high signal to noise).
It is an object of the present invention to overcome or substantially mitigate the above disadvantages.
According to a first aspect of the present invention there is provided a method of detecting an object located within a dynamic scattering media, the method composing:
i) directing a continuous coherent light wave of predetermined wavelength into the media;
ii) detecting dynamically scattered light emerging from the media;
iii) correlating the detected light photons in the time or frequency domain;
iv) determining the presence of an object from analysis of differences between said correlation and the correlation which would arise from photons scattered by the media only; and
v) determining the approximate position of the object within the media from said analysis of the correlation and knowledge of the mean transport path of the light wave of predetermined wavelength within the media.
The term xe2x80x9clight wavexe2x80x9d is not limited to visible light but is to be interpreted as encompassing electromagnetic radiation of any suitable wavelength.
The media may be a turbid media of relatively high density and the object may be more or less dense and more or less viscous than the media. The object may be an object which absorbs and/or reflects the incident light waves.
The invention (from hereon referred to as Diffuse Wave Imaging) incorporates aspects of the known technique of Diffusing Wave Spectroscopy (DWS). DWS is used for sub-micron particle sizing and bulk rheology measurements in dense suspensions or emuisions. DWS is not applicable to the imaging of single objects within turbid media. Furthermore, DWS is not used to identify individual particles. The suppression of photons close to the axis of the auto-correlation traces is treated as a limitation of DWS. The inventors have realised that this suppression of photons which have undergone multiple scattering events can be used to determine the presence of an object, and by taking many measurements, to form an image.
The invention allows images to be obtained using photons which have travelled optical distances greater than 2 photon transport paths. This is opposite to conventional imaging of dense media, which generally removes or filters out light which has been scattered more than once. Since the invention does not require photons which have undergone ballistic and low order scattering, it is suitable for imaging very dense suspensions where ballistic scattering is limited.
The steps (i) to (v) may be iterated either sequentially or simultaneously using coherent light waves of different predetermined frequencies having different mean transport paths within the dynamic scattering media, to thereby obtain further information as to the approximate position of the detected object from the analysis of the respective correlations and knowledge of the respective mean transport paths. For instance, light of three different wavelengths may be used.
The or each iteration of steps (i) to (v) may be repeated, either sequentially or simultaneously, for additional locations within the media, the results then being combined to construct an image of the object within the media. Where only one wavelength of light is used the image will be two dimensional. However, using two or more different wavelengths as mentioned above (which effectively probe into different depths of the media and/or object, enables the construction of a, three dimensional image.
The light emerging from the media may be detected at one or more predetermined scattering angles, preferably a scattering angle of 180xc2x0 and/or 0xc2x0.
The media may be modulated to induce, or enhance, dynamic motion within the media to provide or enhance the required dynamic scattering. Similarly, when the object is an object which is at least partially reflective of the or each light wave, the object may be modulated to enhance phase chances in light reflected therefrom.
The method may included the step of selecting for detection light which has a predetermined component of polarisation. The selection may be accomplished using polarising filters or fibre optic cables which preserve only one particular component of polarisation.
The or each light wave may be passed through a window prior to entering the media, the window being arranged to reflect light which is detected together with light emerging from the media, thereby producing a heterodyne signal. The window may be adjustably displaced relative to the origin of said light wave to allow control of the intensity of the reflected light which is detected. The window may be arranged to cause the reflected light to undergo multiple reflections before being detected, thereby enabling the path length travelled by the reflected light to be controlled.
The method may be performed on a human or animal body to detect the presence and approximate positions, or construct an image of a pathological entity within the body.
According to a second aspect of the present invention there is provided a method of detecting the presence of a pathological entity within the human or animal body, the method comprising:
i) directing a continuous coherent light wave of a first predetermined wavelength into the body;
ii) detecting dynamically scattered light emerging from the body;
iii) correlating the detected light photons in the time or frequency domain; and
iv) determining the presence of a pathological entity from analysis of differences between correlation and the correlation arising from photons scattered by the media surrounding the entity only.
According to a third aspect of the present invention there is provided apparatus for detecting an object located within a dynamic scattering media, the apparatus comprising means for directing a continuous coherent light wave of a predetermined wavelength into the media, means for detecting dynamically scattered light emerging from the media, means for correlating the detected light photons in the time or frequency domain, whereby the presence of an object can be determined from analysis of differences between said correlation and the correlation which would arise from photons scattered by the media only, the approximate position of the object within the media can be determined from said analysis of the correlation and knowledge of the mean transport path of the light wave of predetermined wavelength within the media.
A plurality of detectors may be arranged in an array to provide a series of measurements simultaneously. The array of detectors may be coupled to 3 CCD camera.
Preferably the or each detector is either located adjacent the emitter or is displaced from the emitter and is located on the axis of emission of the emitter
The coherent light producing means produces both visible light and infrared light.
Polarising filters may be located in front of the detection means to select either light with a polarisation perpendicular to the light emitted from the emitter, or to select light with the same polarisation as the light emitted from the emitter.
An object located within a medium may be caused to modulate to increase the contrast of the phase of scattered light, and thereby improve the resolution of the image.
The invention may be used in combination with Time of Flight apparatus to provide imaging over both short and long distances.
Where heterodyne detection is to be used, the coupling means may be provided with a window which will reflect a fraction of the light towards the detection means, thereby providing a heterodyne signal.
The window may be adjustably displaced from the coupling means to allow control of the intensity of reflected light incident at the detection means.
The window may be arranged to cause the reflected light to undergo multiple reflections before being incident at the detection means, thereby allowing the path length traveled by the reflected light to be controlled.
The coupling means and detection means may comprised polarisation preserving optical fibres. The fibres may be mounted so as to be rotatable through 90 degrees, thereby allowing modification of the effective numerical aperture of the fibres.
Preferably the means for producing coherent light comprises a laser.
Preferably four lasers operable at a different wavelength are used concurrently.
Preferably, two of the lasers are arranged to produce orthogonally polarised light, and a polarising beamsplitter cube is provided to couple the light into a polarisation preserving optical fibre.
The coupling means and detection means may comprise optical fibres with terminations located in a probe comprising a cylindrical head.
According to a fourth aspect of the present invention there is provided apparatus for detecting the presence of a pathological entity within the human or animal body, the apparatus comprising means for directing a continuous coherent light wave of a first predetermined wavelength into the body, means for detecting dynamically scattered light emerging form the body, and means for correlating the detected light photons in the time or frequency domain, whereby determining the presence of a pathological entity may be determined from analysis of differences between said correlation and the correlation arising from photons scattered by the media surrounding the entity only.
According to a fifth aspect of the present invention there is provided a method of detecting the presence of an object within a media, the method comprising
i) inducing vibration in the object at a predetermined frequency which does not propagate efficiently within the media;
ii) generating a continuous coherent light wave;
iii) modulating the generated light wave at a second predetermined frequency;
iv) directing the modulated coherent light wave into the media;
v) detecting scattered light emerging from the media;
vi) analysing the detected scattered light for the existence of a beat signal corresponding to the beat frequency between the first and second predetermined frequencies thereby indicating the presence of the object.
The frequency of the induced vibration is selected to correspond To the resonant frequency of the object, or regions within the object to be detected. Such regions may for instance be regions of stress, such as cracks in the object.
The vibrating frequency may be varied until the object, or parts of the object to be detected, resonates, and wherein the size of the resonating object, or region of the object, is determined as a function of the resonating frequency.
An image of the object, or parts of regions of the object to be detected, may be constructed by detecting light scattered from different parts of the object.
The method may be performed on a human or animal body to detect the presence, or construct an image, of a pathological entity within the body.
The invention also provides apparatus for detecting the presence of an object within a media, the apparatus comprising:
i) means for inducing vibration in the object at a predetermined frequency which does not propagate efficiently within the media;
ii) means for generating a continuous coherent light wave;
iii) modulating the generated light wave at a second predetermined frequency;
iv) means for directing thy modulated coherent light wave into the media;
v) means for,detecting scattered light emerging from the media;
vi) means for analysing the detected scattered light for the existence of a beat signal corresponding to the beat frequency between the first and second predetermined frequencies thereby indicating the presence of the object.
The method and apparatus according to the fifth aspect of the invention may be used to investigate properties that cannot be imaged directly such as scaling in an oil pipe or cracks in a metal structure.
The modulation frequency may be used to perform a secondary function on the object. For example, resonance may be used to cause descaling of a pipe whilst an image (by means of measurement of the change in the magnitude of resonance at a single frequency or preferably the change in the resonant frequency) is used to monitor the descaling operation in real time. Similarly, where the methods is used on a human or animal body, the same method may be used to remove or destroy pathological entities such as tumours (wherein the resonance may heat and kill the target), gall stone (wherein the resonance will physically break down the target), kidney stones, and other growths, foreign bodies and abnormalities. A similar method may be used to remove blockages etc. in non-medical applications, such as to remove blockages from target objects such as underwater pipe lines or cables.
Resonance may also be used to modify the target object means of chemical release or activation.
The invention may utilise a naturally occurring source as the source of the investigative wave.
The present invention may be combined with existing detection or imaging systems, such as time of flight systems, CAT, electron resonance etc.
Other possible features of the invention will become apparent from the description below.